The invention relates to the field of institutional hot water heating systems, as would be used in public schools, hospitals, health care centers, spas, nursing homes, hotels, motels, prisons and industry. Institutional hot water systems are distinguished in that there is both a diversified hot water demand and a brief high peak hot water demand. The diversified load means that there are a number of appliances requiring hot water, including sinks, showers, tubs, and dishwashers used periodically. High peak demand appliances include gang showers, commercial washers, whirlpools and therapy tubs.
For many years, hot water was supplied to institutions in the same manner it was supplied for home use that is by a single storage tank with internal heaters having low efficiencies (70% at best). For institutional use, however the storage tank needed to be scaled up to a very large size capable of handling peak demand. This type of system was quite inefficient, as it required keeping a large quantity of water hot during long periods with little demand.
In the late 1940s, two other methods of supplying institutional hot water were developed, the instantaneous heater and the semi-instantaneous heater.
The instantaneous heating system required the same large storage tank with a closed loop recirculation system between the heater and tank in order to maintain proper temperature control.
The semi-instantaneous heater had excellent temperature control without the use of storage, but had to be sized to handle the highest peak demand of the system, even when that peak demand might last for only 10 or 15 minutes per hour. Often, large stratified tanks or accumulator tanks were used in conjunction with semi-instantaneous heaters to solve the high periodic demand load.
As an example, a high school uses between 0 and 25 gallons per minute of 110.degree. F. water in its diversified load, sinks, lavatories and cafeterias, and has a high peak demand load of 40 gpm gang showers that operate 10 minutes every hour. If the diversified load averages 60% of the diversified load capacity during class hours, the total draw per hour will be 60%.times.25 gpm.times.60 min.=900 gallons per hour, plus the peak load 10 min..times.40 gpm=400 gallons per hour for the showers. The total usage will be 1300 gph.
If this load were to be handled with a traditional storage tank installation, that tank would be sized at a minimum of 2000 gallons holding capacity and a 1500 gph recovery rate with an efficiency of 70% at best. If an instantaneous or semi-instantaneous heater were used in the above installation, it would require at least a 1,000,000 BTU per hour heater to furnish the required 1500 gallons per hour hot water with a closed loop recirculation pump to keep the 2000 gallons of water in the tank hot.
Typical prior art institutional heating systems are shown in FIGS. 1 and 2. In FIG. 1, cold water enters the bottom of accumulator 10 and passes to heater 12 from which it passes to the demand appliances or is recirculated back to the bottom of the accumulator. Circulation pump 14 must run continuously to keep the tank water hot. This is the least effective method of providing sufficient hot water because the total flow must pass into the bottom of the tank forcing its stored hot water through the heater. For this reason, the heater does not begin to function until most of the hot water in the accumulator tank is used and cold water enters the heater from the tank. Heating time is therefore lost while the tank is emptying and the heater is idling until it senses cold water.
In FIG. 2, a stratified hot water storage tank 20 is utilized. The cold water supply 26 enters the hot water tank at the bottom, and the hot water output of the system 28 is taken directly from the top of the storage tank. A temperature sensor is provided in the bottom of storage tank 20, and when the sensor determines that cold water is entering the tank due to utilization of hot water, pump 24 is activated to supply cold water to heater 22. The output of heater 22 is then combined with the output of the storage tank.
The system shown in FIG. 2 works well when there is a high draw on the system at or above the heater capacity. If the heater can produce 27 gpm of 110.degree. F. water, and the load requirements are 27 gpm or greater, the pump will run continuously; the heater will operate at full capacity and only the hot water demand above 27 gpm will actually be depleted from the tank. Thus, in the heavy load situation both tank and heater operate at their best combined effectiveness.
A problem arises with the system of FIG. 2 when the demand drops below 27 gpm, which is 50 minutes of every hour during class days, plus nights holidays and weekends. During that time when there is normal demand, whenever a faucet is opened or dishwasher runs, the temperature sensor in the tank will feel cold water and activate the pump. Since 27 gpm circulates through the heater, as little as 10 gpm may go to the faucets while 17 gpm of heated water returns to the tank. This heated water will be sensed by the temperature sensor, and shut off the pump in as little as a minute or two. If the 10 gpm draw continues, the pump and heater could cycle on and off 15 or more times per hour. Such cycling wastes electricity and causes premature pump failure.
In order to solve this problem, a baffle may be installed in the bottom of the storage tank so that the electric sensor does not feel cold water until 30 to 40% of the reserve water is drawn off from the tank. While this solves the problem of the pump cycling on and off, it creates another problem since the tank must be sized 30 to 40% larger to compensate.
Another approach to the problem of supplying hot water over a wide range of demand is disclosed in U.S. Pat. Nos. 3,668,839 and 3,766,974. These patents disclose a hot water storage tank with a cold water supply, an internal heater, and a continuously operating pump to cycle tank water through the heater. Under high demand conditions, hotter tank water is drawn from the upper part of the tank to increase the capacity of the heater.
U.S. Pat. No. 3,705,574 discloses a hot water heating system with separate heater and storage tank. Circulation of water between the heater and tank is controlled by a thermostatic mixing valve which admits cold water when the water passing through the valve exceeds a predetermined temperature.